Refactor beast core, http, tests, and examples:

* Fix warnings
* Port cmake scripts to linux
* Add command line options for running test suites
* Add examples to CMakeLists
* Return std::uint64_t from writer::content_length
* basic_parser::write takes asio::const_buffer instead of pointer and size
* Turn message test back on now that it passes
* Rename to http::headers, use std::allocator, remove http_headers
* http::message::method is now a string
* Refactor to_string for ConstBufferSequence
* Remove chunk_encode from the public interface
* Initialize members for default constructed iterators
* Disallow default construction for dependent buffer sequences

Refactor http::message serialization:

* Serialization no longer creates a copy of the
  headers and modifies them
* New function prepare(), sets Connection, Transfer-Encoding,
  Content-Length based on the body attributes and caller options.
  Callers can use prepare() to have the fields set automatically,
  or they can set the fields manually.
* Use write for operator<<
* Tests for serialization

examples/http_example.cpp

@@ -23,9 +23,10 @@ int main()
     using namespace beast::http;
 
     // Send HTTP request using beast
-    request<empty_body> req({method_t::http_get, "/", 11});
+    request<empty_body> req({"GET", "/", 11});
     req.headers.replace("Host", host + ":" + std::to_string(sock.remote_endpoint().port()));
     req.headers.replace("User-Agent", "Beast");
+    prepare(req);
     write(sock, req);
 
     // Receive and print HTTP response using beast